Wax deposition and rheology: progress and problems from an operator

Abstract

Abstract The deposition and gelation of paraffins pose significant flow assurance risks, especially in offshore developments. Even though there have been numerous studies and breakthroughs in understanding the deposition and gelation phenomena over the years, there still continue to be some challenges that have remained unsolved. A primary issue is modeling deposition accurately in the turbulent flow regime. While some models/simulators are able to predict trends qualitatively, there isn't enough confidence in these predictions to reliably estimate deposition over the long term (order of 1 to 10 years). Another unresolved issue is the determination of the pressure required to restart a gelled pipeline. Various researchers have proposed different approaches, which are contradictory. As an example case, this paper focuses on a highly waxy crude oil with a WAT in the range of 125°F and pour point in the range of 90°F, and highlights some of the problems faced with the measurement techniques, the model development, and verification from an operator's perspective. The paper also describes some of the progress that has been made in these areas of late. Introduction Wax deposition in production pipelines is one of the major flow assurance risks that needs to be considered while developing new fields or maintaining existing operations. The problems associated with wax deposition are reduced productivity, increased pressure drop and the risk of getting a pig stuck during maintenance operations. It is known that the production losses and remediation operations associated with paraffin deposition cost millions of dollars (Oil & Gas J.,2001). Flow assurance risks, including wax deposition and wax gelation, become a bigger concern as the oil industry continues to expand deepwater operations to greater depths and distances in cold environments. Typically, the stock tank oil Wax Appearance Temperature (WAT) is used as a conservative design criterion to protect against wax precipitation in multiphase in-field flowlines. The WAT is determined by a method such as Cross Polar Microscopy (CPM) or Differential Scanning Calorimetry (DSC). While the WAT is an important quantity, the amount of wax precipitated at various temperatures is equally important in determining wax deposition rates and gelation characteristics. The pour point is the usual way of determining the temperature below which the oil will not flow. However, this definition is only based on lowering the temperature of oil under static conditions - i.e., when the structure of the precipitated wax network is not disturbed by shear forces. While such basic measurements such as WAT, pour point and others (including non-Newtonian viscosity) can be measured relatively easily, predicting the deposition rates and pressure requirements for restarting a gelled pipeline are areas where there is scope for improvement. In a previous OTC paper (Venkatesan and Creek, 2007), we discussed the necessity for improved deposition modeling under turbulent flow, and noted the problems with some approaches for modeling deposition. We also discussed some limitation with available measurements. In the current paper, we show the example of a highly paraffinic crude oil and explain the methods used to determine the wax deposition rates and gelation characteristics of the oil. The oil studied has high n-paraffin content (~15% S n-C20+) and, consequently, has high WAT and pour point. Both wax deposition and gelation are expected to pose operational problems. Hence, a detailed wax characterization study was performed to determine expected deposition rates, and requirements for restarting the pipeline after a shutdown. This paper discusses the measurements and modeling, while highlighting some of the current issues/problems in these areas.